Was the initial ambush a result of the French trappers kidnapping...
...that one Indian woman?
Actually, was all of the Indian violence in the film a direct result of the actions of the French trappers or was some of it run of the mill opportunistic Indian looting/pillaging/massacring ?
EDIT: a secondary question would be this: is there any historical basis for French trappers doing something so stupid? Did this ever happen? It seems like a horrifically bad idea to kidnap an Indian girl/woman because of the hellfire that will rain down upon you afterwards.